1. Field of the Invention
The present invention relates to a method and an apparatus for controlling the rotation velocity of an ultrasonic motor which is driven by elastic vibration caused by the piezoelectric effect of a piezoelectric body.
2. Description of the Related Art
In recent years, ultrasonic motors which have a vibration body using a piezoelectric body formed of piezoelectric ceramic or the like for converting electric energy into mechanical energy and are driven by elastic vibration of the vibration body have been a target for attention. The ultrasonic motors are available, for example, in a ring type having a ring-shaped vibration body and a disc type having a disc-shaped vibration body.
Referring to FIG. 9, a disc-type ultrasonic motor 50 will be described. FIG. 9 is a partially cut perspective view of the ultrasonic motor 50. The ultrasonic motor 50 includes a vibration body 3. The vibration body 3 includes a disc-shaped elastic substrate 1 formed of an elastic material such as metal or ceramic, and a disc-shaped piezoelectric body 2 formed of ceramic provided on one of two main surfaces of the elastic substrate 1. On the other main surface of the elastic substrate 1, projections 3a are provided for enlarging the displacement of the vibration body caused by the vibration. The ultrasonic motor 50 further includes a rotation body 6 including a friction body 4 formed of an anti-abrasion material and an elastic body 5 formed of, for example, metal or plastics. The rotation body 6 is provided on the vibration body 3 on the side of the elastic body 5 to stably apply pressure on the vibration body 3 due to a spring or the like (not shown). The friction body 4 enhances the resistance against abrasion and ensures stable contact between the rotation body 6 and the projection 3a.
The piezoelectric body 2 is provided with two sets of driving electrodes (not shown) which are positionally shifted from each other in a circumferential direction by 1/4 of a wavelength of a vibration caused by the vibration body 3. When the two sets of electrodes are respectively supplied with AC voltages having a phase difference from each other by 90.degree., flexural vibration travelling waves are excited in the vibration body 3. The flexural vibration travelling waves have a vertical displacement distribution in the radial direction as is shown in FIG. 10A. FIG. 10B schematically illustrates the displacement distribution of the vibration body 3. There are second or greater order vibration modes in the radial direction and third or greater order vibration modes in the circumferential direction.
When the flexural vibration travelling waves are excited in the vibration body 3, given points on the surface of the vibration body 3 in contact with the rotation body 6 move along an elliptic shape. The vibration body 6 contacts the rotation body 6 only at the crests of the flexural vibration travelling waves, and moves by friction by a horizontal displacement component of the crests of the waves, thereby moving oppositely from the travelling direction of the waves. In other words, the rotation body 6 rotates around a rotation axis 7. The rotation direction of the rotation body 6 can easily be changed by changing the travelling direction of the waves by inverting the sign of the 90.degree. phase difference between the two AC voltages applied to the two sets of driving electrodes.
The operation of the above-described ultrasonic motor 50 will be described in detail.
The two sets of driving electrodes provided on the piezoelectric body 2 are respectively supplied with voltages V.sub.1 and V.sub.2 having a 90.degree. phase difference from each other expressed by Equations (1) and (2). EQU V.sub.1 -V.sub.0 .times.sin(.omega.t) (1) EQU V.sub.2 =V.sub.0 .times.cos(.omega.t) (2)
where
V.sub.0 : the maximum voltage; PA1 .omega.: angular frequency; and PA1 t: time. PA1 .xi..sub.0 : the maximum amplitude of the flexural vibration; PA1 k: the number of waves (=2.pi./.lambda.); and PA1 x: the position of the coordinate in the travelling direction of the waves.
The two sets of driving electrodes are positionally shifted by 1/4 of the wavelength of the vibration caused in the vibration body 3 as mentioned above. Accordingly, due to the application of the voltages V.sub.1 and V.sub.2, two standing waves expressed by Equations (3) and (4) are excited on the piezoelectric body 2. The two standing waves are positionally shifted by .lambda./4 and have a 90.degree. phase difference in terms of time. EQU .xi.1=.xi..sub.0 sin .omega.t cos kx (3) EQU .xi.2=.xi..sub.0 cos .omega.t sin kx (4)
where
Accordingly, the flexural vibration travelling waves .xi. expressed by Equation (5) in the circumferential direction are excited in the vibration body 3. EQU .xi.=.xi..sub.1 +.xi..sub.2 =.xi..sub.0 sin(.omega.t-kx) (5)
From Equation (5), the travelling direction of the waves can be switched simply by changing the phase difference between the voltages V.sub.1 and V.sub.2 to +90.degree. or -90.degree.. Thus, the rotation direction of the vibration body 3 can easily be changed.
FIG. 11A is a view for describing the operation principles behind the ultrasonic motor 50.
A driving force is transmitted from the vibration body 3 to the rotation body 6. The interface between the vibration body 3 and the rotation body 6 shown in FIG. 11A is described approximately by using a simplified linear model, although in practice it has a more complicated shape. When the flexural vibration travelling waves are excited on the vibration body 3, the given points on the surface of the vibration body 3 move along an ellipse having a long axis w and a short axis u as is shown in FIG. 11B. The rotation body 6 contacts the vibration body 3 at the crests of the waves (for example, point A), and is driven by friction by the horizontal displacement component of the vibration body 3, thereby moving in the opposite direction from the travelling direction of the waves at a velocity v expressed by Equation (6). EQU v=.omega..times.u (6)
Since the crests of the waves continuously move, the contact points of the vibration body 3 and the rotation body 6 also move along with time. Thus, the rotation body 6 is continuously driven to smoothly rotate.
FIG. 12 is a block diagram of a conventional circuit 60 for driving and controlling the velocity of the ultrasonic motor 50.
A variable-voltage oscillation circuit 8 generates an AC signal for driving the ultrasonic motor 50. The AC signal from the variable-voltage oscillation circuit 8 is divided into two signals. One signal is shifted in phase as predetermined (by +90.degree. or -90.degree.) by a phase shifter circuit 9 and is sent to a power amplifier circuit 10a. The other signal is directly sent to another power amplifier circuit 10b. The signals are amplified by the power amplifier circuits 10a and 10b respectively to a level which is sufficiently high to drive the ultrasonic motor 50. Then, the waveforms of the signals are shaped by coils 11a and 11b, and then the signals are input to two driving electrode terminals of the vibration body 3. As a result, the flexural vibration travelling waves are excited in the vibration body 3, thereby rotating the rotation body 6.
The variable-voltage oscillation circuit 8 and the phase shifter circuit 9, which operate at a low voltage, are connected to a DC power supply 13a generating a low voltage. The power amplifier circuits 10a and 10b are connected to a DC power supply 13b for generating a driving voltage which is sufficiently high to drive the ultrasonic motor 50. Instead of the power amplifier circuits 10a and 10b, a current amplifier circuit and a voltage boost transformer can be used to amplify the voltages.
An ultrasonic motor generally has resonance characteristics as other piezoelectric devices. Accordingly, as the driving frequency becomes closer to the resonant frequency, the impedance becomes lower and the current flowing through the driving terminal becomes larger. In other words, as the driving frequency becomes closer to the resonant frequency, the vibration amplitude of the vibration body becomes larger, and thus the velocity of the rotation body becomes higher.
In the circuit 60 shown in FIG. 12, the velocity of the rotation body 6 is controlled by adjusting the frequency of the AC signal generated by the variable-voltage oscillation circuit 8 in accordance with a control voltage applied to a control terminal T. The velocity of the rotation body 6 is detected by a velocity detection circuit 12. Practically, the velocity is detected by a rotary encoder attached to the rotation body 6, or based on the vibration amplitude of the vibration body 3 using the principle that the velocity of the rotation body 6 is in proportion to the vibration amplitude of the vibration body 3.
FIG. 13 is a block diagram of another conventional circuit 70 for driving the ultrasonic motor 50.
A signal generated from an oscillation circuit 14 is divided into two signals. One signal is shifted in phase by a phase shifter circuit 15 by a predetermined amount and then is sent to a power amplifier circuit 17a. The other signal is directly sent to another power amplifier circuit 17b. The signals are amplified by the power amplifier circuits 17a and 17b respectively to a level which is sufficiently high to drive the ultrasonic motor 50. Then, the waveforms of the signals are shaped by coils 18a and 18b, and then the signals are input to two driving electrode terminals of the vibration body 3. As a result, the flexural vibration travelling waves are excited in the vibration body 3, thereby rotating the rotation body 6.
The oscillation circuit 14 and the phase shifter circuit 15, which operate at a low voltage, are directly connected to a DC power supply 13c generating a low voltage. The power amplifier circuits 17a and 17b, which require a DC voltage generating a sufficiently high voltage to drive the ultrasonic motor 50, is connected to the DC power supply 13c through an amplitude control circuit 16 for boosting the voltage sent from the DC power supply 13c.
In the circuit 70 shown in FIG. 13, the velocity of the rotation body 6 is controlled by detecting the velocity by a velocity detection circuit 19 and adjusting the amplitude of the driving voltage. The amplitude of the driving voltage is adjusting by changing the voltage sent to the power amplifier circuits 17a and 17b from the DC power supply 13c by the amplitude control circuit 19.
A conventional ring-shaped ultrasonic motor is described in U.S. Pat. No. 4,853,579. The ring-shaped ultrasonic motor, which uses flexural vibration travelling waves having first or greater order vibration modes in the radial direction and third or greater order vibration modes in the circumferential direction, operates by a similar principle and is controlled by a similar velocity control circuit as mentioned above.
A conventional standing wave ultrasonic motor in which the rotation body is loaded by a vibration piece to rotate, also uses a similar velocity control system.
The above-described conventional circuits for driving and controlling the velocity of an ultrasonic motor have the following problems:
Either the frequency of the two driving voltages, the amplitude of the driving voltages, and the phase difference between the driving signals is adjusted to control the velocity of the rotation body 6. In such a system, the adjustment should be done very precisely in order to control the velocity with high accuracy. This requires a highly precise variable-frequency oscillation circuit or a highly precise variable-voltage amplitude circuit, which complicates the circuitry and raises production cost.
For example, a driving apparatus for an ultrasonic motor described in Japanese Laid-Open Patent Publication No. 3-239168 includes a device for varying the driving frequency and a device for varying the driving voltage. However, the device for varying the driving frequency is provided in order to obtain the maximum efficiency of the ultrasonic motor, and thus the driving frequency is set to be around the antiresonant frequency of the ultrasonic motor under predetermined driving conditions. Accordingly, the velocity control is practically performed only by adjustment of the driving voltage.
A driving apparatus for an ultrasonic motor described in Japanese Laid-Open Patent Publication No. 4-222476 includes a device for varying the frequency and a device for varying the applied voltage. These devices are used to constantly drive the ultrasonic motor efficiently at around a resonant frequency. The driving frequency is changed so as to maintain the phase difference between the voltage of the vibration body detected by a vibration state detecting device and the voltage of a signal applied to the piezoelectric body at a constant level. The applied voltage is changed so as to maintain the above-mentioned voltage Of the vibration body (corresponding to the velocity of the rotation body) at a predetermined level. However, the driving frequency is changed to constantly drive the ultrasonic motor at a resonant frequency thereof, and therefore the velocity is practically controlled only by the adjustment of the applied voltage.
In the above-described two conventional ultrasonic motors, the driving or applied voltage should be adjusted highly precisely in order to control the velocity with high accuracy.
In order to control the velocity with high precision using a digital circuit such as a microcomputer, a large number of control bits are required to improve the resolution of the variable frequency and the variable voltage amplitude. This presents the problem of high production cost, which prevents practical use of ultrasonic motors.